Drug-Induced Thyroid Dysfunction
When the medication list is the diagnosis
Amiodarone
Why it disturbs the thyroid. Amiodarone is about 37% iodine by weight, blocks 5′-deiodinase (reducing T4→T3 conversion), is directly toxic to thyroid follicles, and has a half-life measured in months. It can therefore cause hypothyroidism or thyrotoxicosis, even long after it is stopped. Check TSH and free T4 at baseline and every 3–6 months.
In a euthyroid patient on amiodarone, a high-normal or mildly high free T4, a low-normal T3, and a transient TSH rise in the first months are expected drug effects, not disease.
Amiodarone-induced hypothyroidism. The gland fails to escape the iodine-driven Wolff–Chaikoff effect. It is most common in iodine-sufficient regions and in women with underlying Hashimoto thyroiditis (positive TPO antibodies). Treat with levothyroxine and continue amiodarone if the arrhythmia requires it.
| Feature | Type 1 | Type 2 |
|---|---|---|
| Mechanism | Iodine-fueled excess synthesis | Destructive thyroiditis (leak of stored hormone) |
| Underlying gland | Abnormal: nodular goiter or latent Graves | Previously normal |
| Doppler flow | Normal or increased | Absent |
| Radioiodine uptake | Low-normal (can be higher in iodine-deficient areas) | Near zero |
| Treatment | High-dose methimazole ± potassium perchlorate | Prednisone for 1–3 months, then taper |
Managing AIT. Perchlorate (where available) blocks further iodide entry into the gland. Type 2 can appear years into therapy, even after the drug is stopped. Radioiodine fails because the iodine load prevents uptake. Refractory disease or worsening cardiac status is an indication for thyroidectomy. Whether to stop amiodarone is a joint decision with cardiology; its long half-life means stopping brings little short-term benefit.
Lithium and iodine
Lithium. Concentrated by the thyroid, lithium inhibits hormone release; the resulting TSH drive causes goiter and hypothyroidism, more often in women and TPO-positive patients. Rarely it triggers painless thyroiditis with transient thyrotoxicosis. Check TSH before starting and every 6–12 months. If hypothyroidism develops, add levothyroxine and continue lithium. Lithium also causes nephrogenic diabetes insipidus and hyperparathyroidism with hypercalcemia, so monitor calcium as well.
Wolff–Chaikoff effect. A large iodine load (iodinated contrast, amiodarone, povidone-iodine, kelp) transiently blocks organification. Normal glands escape within days, but glands with Hashimoto thyroiditis, a prior partial thyroidectomy or radioiodine treatment, and the fetal or neonatal thyroid may not, producing iodine-induced hypothyroidism. The effect is exploited therapeutically: potassium iodide is given before thyroidectomy and in thyroid storm, always after the thionamide.
Jod-Basedow phenomenon. In a nodular goiter (especially in someone from an iodine-deficient region) or latent Graves disease, an iodine load gives autonomous tissue the substrate to overproduce. Thyrotoxicosis appears weeks after iodinated contrast, with low radioiodine uptake (tracer diluted) and high urine iodine. Treat with a β-blocker, adding methimazole when needed; it often subsides as the iodine clears.
Immunotherapy and targeted drugs
| Drug | Typical thyroid effect | Management |
|---|---|---|
| PD-1/PD-L1 inhibitors (pembrolizumab, nivolumab) | Painless thyroiditis within weeks, then often permanent hypothyroidism; highest risk when combined with ipilimumab | β-blocker, then levothyroxine; usually continue immunotherapy; TFTs every 4–6 weeks |
| Ipilimumab (anti-CTLA-4) | Hypophysitis: low TSH with low free T4 | Check cortisol; glucocorticoids before levothyroxine |
| Interferon-α | Painless thyroiditis, Hashimoto-type hypothyroidism, or Graves; more likely with TPO antibodies | Periodic TSH; treat the specific pattern |
| Alemtuzumab (multiple sclerosis) | Thyroid autoimmunity in about a third, most often Graves, typically 1–3 years later | TSH every 3 months for 4 years |
| Sunitinib, sorafenib (VEGF-receptor TKIs) | Hypothyroidism, sometimes after transient destructive thyrotoxicosis | Monitor TSH; levothyroxine as needed |
Checkpoint-inhibitor thyroiditis. T-cell attack on the thyroid releases stored hormone, producing a brief painless thyrotoxic phase treated with a β-blocker alone, followed by hypothyroidism that is often permanent. Because the endocrine damage is replaceable, the checkpoint inhibitor is usually continued. TKIs likely injure the gland by reducing its blood supply.
A low TSH with a low free T4 on immunotherapy is central hypothyroidism from hypophysitis, not thyroiditis. Check cortisol and replace glucocorticoids before starting levothyroxine to avoid precipitating adrenal crisis.
Levothyroxine interactions and dose needs
How to take it. Take levothyroxine with water on an empty stomach 30–60 minutes before breakfast, or at bedtime, at least 4 hours apart from calcium, iron, bile-acid binders (cholestyramine, colesevelam), sucralfate, and aluminum antacids. PPIs, H. pylori gastritis, and atrophic gastritis impair absorption because tablet dissolution needs gastric acid; celiac disease causes malabsorption.
What raises the requirement. Enzyme inducers (phenytoin, carbamazepine, rifampin, phenobarbital) speed T4 clearance, and oral estrogen raises thyroxine-binding globulin (TBG). In pregnancy, increase the dose by about 25–30% (for example, 2 extra doses per week) as soon as pregnancy is confirmed, then check TSH every 4 weeks through mid-gestation.
- Confirm adherencethe most common cause of a rising TSH
- Review timingfasting, with water, away from food and coffee
- Look for interfering drugscalcium, iron, binders, PPIs, enzyme inducers, estrogen
- Consider GI disease or pregnancyceliac disease, atrophic gastritis, new pregnancy
- Then adjust the doserecheck TSH 6–8 weeks after any change